Pressure swing adsorption is an important gas separation process which is widely used in the process and manufacturing industries. Pressure swing adsorption is used for recovering high-purity gas products from crude process gas streams, for example in hydrogen production, or as an alternative to hauled-in atmospheric gas products or onsite cryogenic air separation processes. The pressure swing adsorption process has been highly developed for the separation of a wide variety of gas mixtures including, for example, the separation of air to provide oxygen and nitrogen products. For smaller product volumes in air separation applications, pressure swing adsorption processes may use a single adsorbent bed and one or more gas storage tanks to provide a constant product flow as well as gas for repressurization and purge. At higher product volumes, multiple adsorbent beds operating in parallel with overlapping cycles are used to generate a constant product gas flow as well as provide gas for repressurization and purge.
Pressure swing adsorption processes can be operated wherein the maximum and minimum cycle pressures are both superatmospheric, wherein the maximum cycle pressure is superatmospheric and the minimum cycle pressure is atmospheric, wherein the maximum cycle pressure is superatmospheric and the minimum cycle pressure is subatmospheric, or wherein the maximum cycle pressure is near atmospheric and the minimum cycle pressure is subatmospheric. The latter two processes have been described in the art as vacuum-pressure swing adsorption (VPSA) and vacuum swing adsorption (VSA). For the purposes of the present disclosure, the generic term xe2x80x9cpressure swing adsorptionxe2x80x9d or PSA will be used to describe any cyclic gas adsorption process which utilizes the effect of pressure on adsorbent capacity to separate gas mixtures. The pressures utilized in a generic PSA process can be superatmospheric, subatmospheric, atmospheric, or combinations thereof.
PSA process technology has been improved significantly over the past decade. Sophisticated process cycles and improved adsorbents have led to more efficient and economical operating PSA plants, particularly for the separation of air, the recovery of hydrogen and carbon monoxide from synthesis gas, and the recovery of hydrogen and light hydrocarbons from gas streams in refineries and petrochemical plants. Further improvements are desirable and continue to be pursued by users of PSA technology.
Two important measures of PSA process performance are the amount of adsorbent required for a given production rate and the percent recovery of the desired product from the feed gas mixture. A known method to reduce the adsorbent requirement is to decrease the cycle time with the pressure envelope held constant. A decrease in cycle time, however, may have a negative impact on recovery. Also, reductions in cycle time may lead to severe problems caused by resulting high gas velocities, including high pressure drop, fluidization, and attrition of the adsorbent material. Therefore, a method is needed to select optimum operating conditions for PSA systems so that an appropriate tradeoff can be achieved between the low adsorbent requirement associated with fast cycles and the potential negative effects associated with fast cycles. The present invention, which is described below and defined by the claims which follow, provides a simple method to achieve this tradeoff.
The invention relates to a pressure swing adsorption process which comprises introducing a feed gas mixture into an inlet of an adsorber vessel during a feed period, wherein the feed gas mixture contains a more strongly adsorbable component and a less strongly adsorbable component and the adsorber vessel contains a bed of adsorbent material which selectively adsorbs the more strongly adsorbable component, and withdrawing a product gas enriched in the less strongly adsorbable component from an outlet of the adsorber vessel during at least a portion of the feed period, wherein a dimensionless cycle-compensated mass transfer coefficient defined as K tfeedVads/Vfeed is maintained in the range of about 23 to about 250, where K is the linear driving force mass transfer coefficient for diffusion of the more strongly adsorbable component in the adsorbent closest to a product end of the bed of adsorbent material, tfeed is the duration of the feed period, Vads is the empty volume of a section of the adsorber vessel which contains the bed of adsorbent material, and Vfeed is the volume of the feed gas mixture introduced into the inlet of the adsorber vessel during the feed period, and wherein Vfeed is defined as NRT/Pads, where N is the number of moles of the feed gas mixture introduced into the inlet of the adsorber vessel during the feed period tfeed, R is the universal gas constant, T is the average absolute temperature of the feed gas mixture at the inlet of the adsorber vessel, and Pads is the absolute pressure of the feed gas at the inlet of the adsorber vessel. The more strongly adsorbed component may be nitrogen and the less strongly adsorbed component may be oxygen.
The value of K tfeedVads/Vfeed may be maintained in the range of about 23 to about 100. The adsorbent material may comprise one or more zeolites, with or without binder material, selected from the group consisting of CaA, NaX, CaX, BaX, LiX, NaLSX, CaLSX, BaLSX, and LiLSX zeolites.
The more strongly adsorbed component may be carbon monoxide and the less strongly adsorbed component may be hydrogen. In this embodiment, K tfeedVads/Vfeed may be maintained in the range of about 66 to about 250. The adsorbent material may comprise one or more zeolites, with or without binder material, selected from the group consisting of CaA, NaX, CaX, BaX, LiX, NaLSX, CaLSX, BaLSX, and LiLSX zeolites.
Typically, the duration of the feed period is in the range of about 7 to about 120 seconds and the adsorbent material comprises particles with an average particle diameter in the range of about 1.2 to about 1.6 mm. More specifically, the duration of the feed period may be in the range of about 3 to about 60 seconds and the adsorbent material may comprise particles with an average particle diameter in the range of about 0.8 to about 1.2 mm.
The duration of the feed period may be in the range of about 0.25 to about 30 seconds and the adsorbent material may comprise particles with an average particle diameter in the range of about 0.3 to about 0.8 mm.
The process may further comprise a purge period during which a purge gas is introduced into the adsorber vessel and passed through the bed of adsorbent material to desorb the more strongly adsorbed component, wherein the value of (xcex94P/P)purge is maintained below about 0.3, where xcex94P is the pressure drop across the bed of adsorbent material at the end of the purge period and P is the minimum absolute pressure in the bed of adsorbent material at the end of the purge period.
The bed of adsorbent material may comprise two or more adsorbents.
In another embodiment, the invention includes a method of operating a pressure swing adsorption process which comprises:
(a) introducing a feed gas mixture at a feed gas flow rate into an inlet of an adsorber vessel during a feed period, tfeed, wherein the feed gas mixture comprises a more strongly adsorbable component and a less strongly adsorbable component and the adsorber vessel contains a bed of adsorbent material which selectively adsorbs the more strongly adsorbable component, and withdrawing a product gas enriched in the less strongly adsorbable component from an outlet of the adsorber vessel during at least a portion of the feed period;
(b) depressurizing the adsorber vessel by withdrawing a depressurization gas therefrom;
(c) purging the bed of adsorbent material during a purge period in which a purge gas is introduced at a purge gas flow rate into the adsorber vessel and passed through the bed of adsorbent material to desorb the more strongly adsorbed component; and
(d) repeating (a) through (c) in a cyclic manner.
The operation of the pressure swing adsorption process may be controlled by selecting a desired value of a dimensionless cycle-compensated mass transfer coefficient defined as K tfeedVads/Vfeed and adjusting the feed gas flow rate, the duration of the feed period, or both the feed gas flow rate and the duration of the feed period to maintain the desired value of K tfeedVads/Vfeed, where K is the linear driving force mass transfer coefficient for diffusion of the more strongly adsorbable component in the adsorbent closest to a product end of the bed of adsorbent material, tfeed is the duration of the feed period, Vads is the empty volume of a section of the adsorber vessel which contains the bed of adsorbent material, and Vfeed is the volume of the feed gas mixture introduced into the inlet of the adsorber vessel during the feed period, and wherein Vfeed is defined as NRT/Pads, where N is the number of moles of the feed gas mixture introduced into the inlet of the adsorber vessel during the feed period tfeed, R is the universal gas constant, T is the average absolute temperature of the feed gas mixture at the inlet of the adsorber vessel, and Pads is the absolute pressure of the feed gas at the inlet of the adsorber vessel.
The desired value of K tfeedVads/Vfeed may be in the range of about 23 to about 250. In this embodiment, the more strongly adsorbed component may be nitrogen and the less strongly adsorbed component may be oxygen. The desired value of K tfeedVads/Vfeed may lie between about 23 and about 100. The adsorbent material may comprise one or more zeolites, with or without binder material, selected from the group consisting of CaA, NaX, CaX, BaX, LiX, NaLSX, CaLSX, BaLSX, and LiLSX zeolites.
The more strongly adsorbed component may be carbon monoxide and the less strongly adsorbed component may be hydrogen. In this embodiment, the desired value of K tfeedVads/Vfeed may lie between about 66 and about 250. The adsorbent material may comprise one or more zeolites, with or without binder material, selected from the group consisting of CaA, NaX, CaX, BaX, LiX, NaLSX, CaLSX, BaLSX, and LiLSX zeolites.
The purge gas flow rate may be controlled such that (xcex94P/P)purge is maintained below about 0.3, where xcex94P is the pressure drop across the bed of adsorbent material at the end of the purge period and P is the minimum absolute pressure in the bed of adsorbent material at the end of the purge period.
The bed of adsorbent material may comprise two or more adsorbents.